The petroleum source is drying up, and at the same time, environmental problems such as global warming from consumption of fossil fuel have increasingly become serious. Thus, a fuel cell receives attention as a clean power source for electric motors that is not accompanied with the generation of carbon dioxide. The above fuel cell has been widely developed, and some fuel cells have become commercially practical. When the above fuel cell is mounted in vehicles and the like, a polymer electrolyte fuel cell comprising a polymer electrolyte membrane is preferably used because it easily provides a high voltage and a large electric current.
The above polymer type fuel cell comprises a pair of electrodes consisting of a fuel electrode and an oxygen electrode, and a polymer electrolyte membrane capable of conducting ions, which is located between the electrodes. Each of the above fuel and oxygen electrodes has a backing layer and a catalyst layer, and the above polymer electrolyte membrane is sandwiched between the above catalyst layers of both the electrodes. The above catalyst layer comprises catalyst particles that are formed by unifying by ion conducting binders, catalysts such as Pt supported by catalyst carriers.
When reducing gas such as hydrogen or methanol is introduced into the fuel electrode of the above polymer electrolyte fuel cell, the above reducing gas reaches the above catalyst layer through the above backing layer, and protons are generated by the action of the above catalyst. The protons transfer from the above catalyst layer to the catalyst layer of the above oxygen electrode through the above polymer electrolyte membrane.
When oxidizing gas such as air or oxygen is introduced into the above oxygen electrode while introducing the above reducing gas into the above fuel electrode, the above protons are reacted with the above oxidizing gas by the action of the above catalyst in the catalyst layer on the side of the above oxygen electrode, so as to generate water. Thus, electric current is obtained by connecting the fuel electrode with oxygen electrode by a conductor.
Previously, in the polymer electrolyte fuel cells, a perfluoroalkylene sulfonic acid polymer (e.g., Nafion (product name) manufactured by DuPont) has been widely used for the above polymer electrolyte membrane and the ion conducting binder in the above catalyst layer. The perfluoroalkylene sulfonic acid polymer is sulfonated, and accordingly it has an excellent proton conductivity. Moreover, the compound also has a chemical resistance as a fluorocarbon resin. However, the compound has a problem in that it is extremely expensive.
Thus, in recent years, a low-priced material that does not contain fluorine in its molecular structure or contains a reduced amount of fluorine has been proposed. For example, the specification of the U.S. Pat. No. 5,403,675 discloses a polymer electrolyte membrane comprising sulfonated rigid polyphenylene. The sulfonated rigid-rod polyphenylene described in the above specification is obtained by reacting a polymer obtained by polymerizing an aromatic compound having a phenylene chain with a sulfonating agent, so as to introduce a sulfonic acid group into the polymer.
However, it is difficult for the polymer electrolyte fuel cell comprising a low-priced material such as the above sulfonated rigid polyphenylene to have the same power generation efficiency as in the case of using the above perfluoroalkylene sulfonic acid polymer.